Department of Mechanical Engineering ME 322 Mechanical Engineering

Department of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lect 27 b Jet Aircraft Propulsion

Aircraft Propulsion • Thrust produced by increasing the kinetic energy of the air in the opposite direction of flight • Slight acceleration of a large mass of air – Engine driving a propeller • Large acceleration of a small mass of air – Turbojet or turbofan engine • Combination of both – Turboprop engine 2

Aircraft Gas Turbine Engines Turboprop Small commuter planes Turbojet high speeds 3 Turbofan Larger passenger airliners

The Turbojet Ideal Turbojet Pressure drop with acceleration Ram effect - pressure rise with deceleration a-1 Isentropic increase in pressure (diffuser) 1 -2 Isentropic compression (compressor) 2 -3 Isobaric heat addition (combustion chamber) 3 -4 Isentropic expansion (turbine) 4 -5 Isentropic decrease in pressure with an increase in fluid velocity (nozzle) 4

The Turbojet with an Afterburner The turbine exhaust is already hot. The afterburner reheats this exhaust to a higher temperature which provides a higher nozzle exit velocity 5

Turbojet Irreversibilities Isentropic efficiencies - Diffuser - Compressor - Turbine - Nozzle Fluid Friction effects - Combustion chamber 6

The Turbojet Model First Law analysis of the components in the cycle Combustion is replaced with a heat transfer Air is the working fluid throughout the complete cycle 7

Turbojet Performance There is no net power output of the turbojet engine. Therefore, the idea of net power and thermal efficiency are not meaningful. In turbojet engines, performance is measured by, • Propulsive Force (Thrust) – The force resulting from the velocity at the nozzle exit • Propulsive Power – The equivalent power developed by the thrust of the engine • Propulsive Efficiency – Relationship between propulsive power and the rate of kinetic energy production 8

Turbojet Performance Propulsive Force (Thrust) in (a) exit (5) In this equation, the velocities are relative to the aircraft (engine). For an aircraft traveling in still air, 9 Propulsive Power The power developed from the thrust of the engine

Turbojet Performance – Efficiencies Overall Efficiency Kinetic energy production rate Thermal Efficiency Thermal power available from the fuel Propulsive Efficiency Propulsive power Kinetic energy production rate 10

Turbojet Example Given: A turbojet engine operating as shown below Find: (a) The properties at all the state points in the cycle (b) The heat transfer rate in the combustion chamber (k. W) (c) The velocity at the nozzle exit (m/s) (d) The propulsive force (lbf) (e) The propulsive power developed (k. W) (f) The propulsive efficiency of the engine 11

Turbojet Example Note: An array position of [0] is allowed in EES! 12

Turbojet Example Strategy: Build the property table first. This will require some thermodynamic analysis. Consider each component in the cycle. Diffuser 13

Turbojet Example Compressor Turbine Combustion Chamber 14

Turbojet Example Nozzle At this point, the property table is complete! 15

Turbojet Example Now, we can continue with the rest of thermodynamic analysis. Combustion Heat Transfer Rate 16 Nozzle – Exit Velocity

Turbojet Example Now the propulsive parameters can be calculated, 17

Turbojet Example Solution (Key Variables): 18

Turbojet Example – Analysis How is the energy input to this engine distributed? excess thermal energy transfer kinetic energy production rate 19